Gas station without pumps

2021 March 23

Vaccine vested!

Filed under: Uncategorized — gasstationwithoutpumps @ 13:41
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Last Friday marked two weeks since I got my second shot of the Moderna COVID-19 vaccine, so I’m now as immunized as I’m going to get against the SARS-COV-2 virus. My wife got her second shot of the Pfizer vaccine, so we are both fully immunized.

We can now go wild! We’re planning to take public transit! to Berkeley! Our current plan is to take the Highway 17 Express (currently half price), then the 500 Express light rail to Berryessa, then BART to Berkeley. I was just getting used to Warm Springs as the bus connection, instead of Hayward BART, and now Berryessa is the transfer point. Who knows—before I die BART may get all the way to Diridon station!

If we somehow miss the 500 express, we can take the Amtrak Capitol Corridor to Berkeley instead, but Amtrak does not take the Clipper Cards (though the supposedly sell them on the Capitol Corridor).  The scenery along the Amtrak tracks is a bit more interesting than along the BART tracks, though.

I got my grading for Winter quarter done last night (two days before the grading deadline), so I finally get a “weekend” after 10 weeks of working 7 days a week.  I do have to put together my syllabus and set up Canvas for my two classes that start on Monday, though, so I can’t get a full week of break.

For my “weekend” I started by putting a new bike computer on my bike.  The old Cateye Enduro 8 finally failed (replacing the battery didn’t fix it), and I bought a new Cateye Velo 7 to replace it, as the closest current equivalent.  The cables and brackets seem cheaper and flimsier than the old ones, so I don’t expect this bicycle computer to last as long.  Most of the bicycle computers on the market seem to be wireless ones, but I really don’t like the idea of having twice as many batteries to replace, shorter battery lifetimes, and lower reliability of wireless units.  I considered getting a cheap Chinese bicycle computer that had a fancier display than the Velo 7, but decided to stick with a name brand that I know has been pretty reliable.

I also took apart the lawn mower to see it if is fixable.  As always, it took me a long time to clear out the grass packed into the recessed screw holes to get the cover off, and a long time to vacuum all the grass out of the interior of the mower.  When I did finally get access to the motor and electronics, I determined that the bridge rectifier had failed again—this time with a short circuit instead of an open circuit.  I’ll buy another GBPC5010-G‎ 50-Amp bridge rectifier, and see if this one lasts a little longer.

2018 January 21

Electric lawnmower failed again

Filed under: Uncategorized — gasstationwithoutpumps @ 15:25
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My Black and Decker electric lawnmower (model MM1800 type 1) failed again in December, and I finally got around to taking the top off today to look at it.  The screws were still a pain to get out, but not as bad as last time, as I now knew to use a vacuum cleaner and a screwdriver to clear out the grass before trying to unscrew the screws.  There was not nearly as much buildup of grass inside the mower this time, so a quick pass with the vacuum cleaner was enough to clear it.

The problem this time was not the bridge rectifier, which seemed to be behaving properly with about 0.55V drop on each of the diodes at the 1mA test current used by my multimeter.  The switch and power connections were all working correctly, so I think the problem now is that the brushes for the motor have worn down to the point where they no longer make contact with the commutator.

A new pair of brushes for the mower costs only $9 (, so I think I’ll replace the brushes and see if that fixes the mower.

2017 May 27

Electric lawnmower repair

Filed under: Uncategorized — gasstationwithoutpumps @ 14:21
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I have a Black and Decker electric lawnmower (model MM1800 type 1), which I have used for a few years, ever since my previous electric lawnmower failed.  A few weeks ago, the lawnmower failed while I was mowing the waist-high grass in the backyard (a heavier load than the mower is designed for).  The failure mode was an interesting one, though—it blew the circuit breaker for the outdoor outlet.  I figured that some of the wet grass had gotten into motor compartment and shorting out the circuit, so I left the mower in the sun for a couple of hours to dry out and tried again.  It still blew the circuit breaker, so I put the mower away until I had time to work on it.

Last weekend, I took the cover off the mower to see if I could see an obvious short circuit.  This looked like it would be an easy job, as the cover is just held down by 6 screws, but the screws are recessed, and there was so much impacted grass built up over them that it took half an hour or more to dig out the grass enough to turn the screws.  One I got the cover off, the amount of grass inside the motor compartment was scary—after dumping out about 200 cubic inches of grass, I spent half an hour with a vacuum cleaner and a screwdriver cleaning off the stuck-on grass clippings.  Here is what it looked like after cleaning:

The mower is very simple, and looks pretty good after all the grass clippings have been removed. All those spaces between the reinforcing ribs were packed solid with grass clippings.

I thought that perhaps the huge buildup of grass clippings had been causing the short circuit, but after removing all the grass, I saw no signs of blackened wires or other evidence of arcing. I also disassembled the switch handle to look for evidence of shorts there. Again, there was nothing visible. I tried powering up the mower while it was open—still the circuit breaker blew instantly.

The lawnmower design is a simple one:

The motor driving the blade is a large brushed DC motor. The switch either provides power to the motor (on) or shorts it out (off) to brake the motor. (Schematic fixed 2018 June 6)

There is easy access to the wiring.

Here is what the rectifier looks like in situ. The four wires are on slide-on connectors, and the rectifier is screwed to a small metal plate that acts as a heat sink.

I got out my cheap ohmmeter and started checking continuity and measuring resistances. I quickly determined that the switch was working correctly, and the the motor itself did not seem to be shorted, but the bridge rectifier was acting as a short across the AC inputs, even with no load connected. I then looked online and found a site that said that the bridge rectifier was one of the most commonly failing parts for the Black and Decker mowers, and that Black and Decker charged ridiculous amounts for replacements. They also said that the part was a 15A, 400V bridge rectifier in a standard package. The one in my mower was labeled GBC2504, which is a 25A, 400V bridge rectifier (so, for that matter, is the own shown on the web page that claimed it was a 15A one). I ordered a 50A, 1000V bridge rectifier from Digikey, which cost $1 more than the closest equivalent to the one Black and Decker used, but even with shipping was half the price of the list price of the poorer rectifier from B&D. Based on the derating curves on the data sheet, the 50A bridge rectifier should be able to run about 30°–40°C hotter before it fails. My only concern is that this might be too good—that B&D was using the rectifier as a cheap point of failure at high temperatures. With the better rectifier the motor might overheat and burn out before the rectifier fails, and a replacement motor is not a $3.50 part.

This morning I replaced the bridge rectifier with the new one, being careful to clean the metal plate and put on new thermal grease to get good thermal contact to the heatsink. The mower started up right away and gave me no problems with mowing the front lawn. I’ll try tackling the waist-high grass tomorrow, but today I need to do some grading. (I have a homework set and something like 22 redone lab reports to grade during this long weekend—I think I’ll be able to clear that this weekend before getting more redone reports on Wednesday.)

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